This invention relates to an automatic focusing device of camera having a movement prediction feature of focalizing the lens on a moving object.
There have been proposed various automatic focusing devices having a so-called movement prediction feature of focalizing the shooting lens on a moving object by detecting the amount of movement of the object along the optical axis of the lens, predicting the location of the image plane of the object after a predetermined time and driving the lens to the predicted image plane to focus the lens on the object.
U.S. Pat. Nos. 4,860,045 and 4,967,224 disclose a method of predicting the movement of the object in the release time lag (the time lag of driving the lens, the reflector, etc.) and correcting the focal point of the lens on the basis of the change in the defocus of the camera.
U.S. Pat. No. 5,208,625 assigned to the assignee of the present patent application also discloses a method of predicting the movement of the object by detecting the amount of movement of the object.
A camera having such an movement prediction feature is normally so designed as to judge the moving object to be stationary if the detected amount of movement is smaller than a predetermined value (threshold level of judging for movement) in order to avoid falsely predicting the movement of a stationary object, which is non-existent.
Then, the camera normally does not operate for movement prediction.
Generally speaking, however, none of such known devices are free from errors in detecting the amount of movement of the object due to computational errors, noises appearing on the sensor and other factors.
Because of such errors in detecting the amount of movement of the object, a relatively high threshold level should be selected for the camera for judging on the movement of the object to accommodate the errors. Otherwise, the camera can falsely predict an nonexistent movement of a stationary object and the lens can get out of focus as it hunts for the illusionary moving object back and forth.
If, on the other hand, the threshold level is too high, the camera can mistake a moving object for a stationary object to neglect its mission of predicting the movement of the object.
While it is a general practice to increase the frequency of detecting the amount of movement of the object, thereby minimizing the detection error in order to select a low threshold level for the camera for judging on the movement of an object and, at the same time, minimize the error of mistaking a stationary object for a moving object, a complicated system is then required for controlling the predicting operation including computations for processing the detected data.